1. Field of the Invention
The present invention relates to a printing apparatus and printing method and, particularly to a printing apparatus including an inkjet full-line head and a method for sorting print media printed by the apparatus.
2. Description of the Related Art
In general, an inkjet printing apparatus (to be referred to as a printing apparatus hereinafter) prints an ink dot by attaching ink as a droplet to a print medium such as paper. Recently, the technological advance of arrayed nozzle integration enables the manufacture of a high-density, long print width printhead. Such a printhead is generally called a full-line printhead, and can complete an image by one printing scan in a wide printing region corresponding to the printhead.
As the print width of the head becomes longer and the nozzle density becomes higher, the number of nozzles tends to increase. As the number of ink types used increases, the total number of nozzles also increases. In some apparatuses, the total number of nozzles becomes several tens of thousands or several hundreds of thousands. However, a larger number of nozzles make it difficult to completely manage the states of the respective nozzles and keep the discharge states of all nozzles normal. Many factors which disturb normal ink discharge are conceivable, including attachment of dust such as paper dust or dirt to the vicinity of the nozzle, attachment of ink mist, an increase in ink viscosity, mixing of air bubbles and dust into ink, and a nozzle failure.
These factors results a discharge failure such as an ink discharge failure or discharge warp, causing an image printing problem such as generation of a stripe. If such a discharge failure occurs during the printing operation, it needs to be detected quickly. However, if the printing apparatus is stopped and recovery such as suction is performed, time is taken until the printing operation is restarted, decreasing the printing efficiency. This arouses demand for confirming printed products later depending on the state of a discharge failure so that they can be sorted into non-defectives and defectives, and continuing the printing operation itself.
To solve these problems, for example, Japanese Patent No. 2931784 discloses a method of, when the printing state is determined to be improper, overlaying and printing a predetermined image on a printed image to substantially invalidate the printed image, accumulating the printed product in the same stacker as that for normal printed products, continuing the printing operation.
Discharge failures that occurred during the printing operation include an accidental discharge failure which is naturally recovered within a short period of time even if it occurs, and a discharge failure which continues once it occurs. For example, even if a discharge failure occurred by an air bubble in ink, the air bubble may be discharged from the orifice depending on the type and size of air bubble, and the discharge failure may be recovered in a short period of time. Also, even if dust is attached to a nozzle and a discharge failure occurs, the dust may be naturally removed depending on the type and size of dust, and the discharge failure is naturally recovered in a short period of time.
Depending on a print image, even if a discharge failure occurs in regard to an ink among a plurality of inks used in the printhead, a normal printed product may be output. For example, in an image in which the density change of each color component is complicated, a problem such as density unevenness arising from a discharge failure is not easily visually recognizable, and the printed product is easily acceptable as a normal one, compared to a case in which the density of a print image is hardly changed and an image is printed at a uniform density. Hence, when a continuous discharge failure occurs, a printed product in which degradation of the image quality is visually recognized is highly likely to be output soon or later. However, when an accidental discharge failure which will be naturally recovered in a short period of time occurs, non-defective printed products and defective printed products may be output and coexist before and after the occurrence of the discharge failure. In this case, it is better to select only non-defective printed products from the output printed products.
However, in a case where the quality of a printed product is accurately determined automatically by an apparatus for each print image, the apparatus control becomes complicated, and it becomes difficult to perform the control at high accuracy. In a situation in which non-defective printed products and visually recognizable defective printed products coexist, the operator may want to reliably make a final judgment by visual check. Also, when a discharge failure is detected, the method disclosed in Japanese Patent No. 2931784 overlays and prints an image representing a defective on a printed product. Even if this method can reduce the apparatus stop time, it wastes a non-defective printed product as a defective.